Let us consider that the Time Devourer was a direct response to Lavos's defeat at the hands of Crono, Marle, Lucca, Ayla, Robo, Frog, and Magus. The very livestock Lavos was hoping to cultivate ended up biting back and pissing it off. Outside of a stable time loop (which as far as something this big goes, Chrono Trigger seems too smart to rely on. I mean cloning an egg in the lost sanctum sure but the freakin entity?), it just isn't possible.
Lavos destroys the world in 1999
Crono party finds out
Lavos is defeated in 1999
Lavos merges with its selves and becomes the Time Devourer
Time Devourer drags the frozen flame's arbiter (Schala) and the mammon machine into the DBT (which is weird because CTDS implies she was there before it became this, when CC implies after but semantics.)
Mind, Schala does not merge with Lavos until *after* it has been defeated. Lavos does this as a direct result. (After, in this case, being a relative term: Lavos exists four-dimensionally, so interacting with 12,000 BC but being elsewhen would be for it like reaching for something to your left or right.) Otherwise, she is lost (or possibly dead) in the ocean palace in the timeline where Crono and party interfered, and moreso survived in the one where they were never involved, in no danger of Lavos whatsoever.
In terms of relative cause-and-effect... she couldn't have been the entity that started guiding them along.